1. Field of the Invention
The present invention relates to a process for preparing pentafluorodichloropropanes, in particular, 1,1,1,2,2-pentafluoro-3,3-dichloropropane (hereinafter referred to as "R-225ca") and 1,1,2,2,3-pentafluoro -1,3-dichloropropane (hereinafter referred to as "R-225cb") which are substitutes for industrially important 1,1,2-trichloro -1,2,2-trifluoroethane and have less influence on the global environment.
2. Description of the Related Art
Hitherto, R-225ca and R-225cb have been technically prepared by reacting tetrafluoroethylene (hereinafter referred to as "TFE") and dichlorofluoromethane (hereinafter referred to as "R-21") at a reaction temperature of 15 to 100.degree. C. in the presence of a catalyst comprising anhydrous aluminum chloride (cf. U.S. Pat, No. 2,462,402, J. Amer. Chem. Soc., 71, 979, and Collect. Czechoslov. Chem. Commun., 36, 1867).
However, if TFE and R-21 are reacted in the presence of the same catalyst, namely anhydrous aluminum chloride at 0.degree. C., 2,2-dichloro-1,1,1,3,3-pentafluoropropane (hereinafter referred to as "R-225aa") is produced (cf. Japanese Patent Kokai Publication No. 209824/1990).
From the above facts, it is highly expected that R-225aa will be by-produced when R-225ca and R-225cb are prepared by reacting TFE and R-21 in the presence of the catalyst comprising anhydrous aluminum chloride. In fact, analysis of the product of this reaction reveals that R-225aa is contained together with R-225ca and R-225cb.
Accordingly, the by-production of R-225aa is unavoidable when R-225ca and R-225cb are prepared by the above conventional process, and a yield and selectivity of the desired compounds are not increased. Therefore, the conventional process is uneconomical in the commercial scale production of R-225ca and R-225cb.
When R-225aa is removed from a mixture of R-225ca, R-225cb and R-225aa to obtain pure R-225ca or a pure mixture of R-225ca and R-225cb, rectification, which is a known purification procedure, cannot effectively and economically separate R-225aa, since R-225aa has a boiling point (51.degree. C.) which is very close to that of R-225ca (51.1.degree. C.).
At present, it is hot certain whether or not R-225aa will be tested for its safety or other properties. Therefore, it is desirable to suppress the by-production of R-225aa as much as possible.